The present invention relates generally to adhesive compositions and more specifically to graft copolymers of starch and vinyl monomers which may be utilized as binding materials in applications including but not limited to paper coating compositions. Specifically, the invention provides improved aqueous polymeric dispersions comprising a graft copolymer of a thinned, gelatinized starch and one or more vinyl grafting monomers, said vinyl grafting monomers comprising at least 10% 1,3-butadiene by weight. The invention further provides such aqueous polymeric dispersions wherein the grafting monomers comprise a vinyl monomer other than 1,3-butadiene in addition to 1,3-butadiene.
It is well known to treat paper and other similar substrates with coating compositions in order to confer improved properties such as gloss, smoothness and printability. In general, paper coatings are comprised of two major components: pigment and binder. Pigment is generally comprised of kaolin clay, either alone or in combination with other pigments such as titanium dioxide, calcium carbonate and plastic pigment. The functions of the pigment in the coating include filling in the irregularities of the paper surface and providing gloss, brightness and ink absorbency. The binder functions to hold the pigment particles together and to bind them to the substrate surface. Natural polymers such as casein, protein and starch are commonly used as the pigment binder in coating formulations. Among these polymers, starch is by far the most important pigment-coating adhesive, primarily because of its low cost. Starch has good binding properties and due to its hydrophilic character, contributes water-holding properties to the coating which dramatically affect the performance of the coating. Paper coated with coating formulations comprising starch as a binder, however, is somewhat lacking in plasticity and gloss after it has been finished.
Rubber latexes and various other synthetics are also used as binders in paper coating compositions. Paper coating materials comprising styrene-butadiene latex are well known in the art and are characterized by low viscosity and by providing improved gloss and softer sheet properties to papers to which they have been coated. Latex, however, is expensive and has poor water holding properties.
Blends of starch and synthetic latex materials are used as binding materials to address the respective deficiencies of the two materials. The use of such blends, particularly blends of modified starch materials with latex materials, provides the combination of advantageous qualities of each of the two types of binder materials. One disadvantage of such blends, however, is mottle wherein differential migration of the binders during the paper drying process produces spots and lack of uniformity on the surface of treated paper. Different areas of the paper surface then accept printing ink differently which can lead to a mottled appearance of the final printed paper.
As one approach to providing improved coating binders, it is known in the art to form graft copolymers of starch and synthetic monomer materials. Brockway, U.S. Pat. No. 3,095,391 teaches the preparation of graft copolymers of granular unpasted starch and a wide variety of vinylic monomers utilizing initiators selected from the group consisting of hydrogen peroxide, organic peroxides, hydroperoxides and sodium hypochlorite. The graft copolymers are said to have utility in textile finishing and sizing as well as in paper making as binders and retention aids. The patent teaches that the pasting properties of the original starch are modified by the grafting procedure and that when the engrafted polymer is hydrophobic, the pasting property of the granular starch is inhibited such that pasting in water is impossible unless the engrafted polymer content is low.
Borunsky, U.S. Pat. No. 3,138,564 discloses a process for utilizing ozone or oxygen to initiate grafting of vinyl monomers onto polysaccharide materials. The reference specifically discloses one example in which styrene and 1,3-butadiene are said to be grafted onto oxidized granular starch. No disclosure is made with respect to grafting efficiency or directed to use of the grafted materials in adhesive or paper coating applications.
Brockway, et al., U.S. Pat. No. 3,061,471 teaches the desirability of gelatinizing starch prior to grafting with monomers in order to produce stable aqueous dispersions with properties similar to those of latexes of other polymers. This patent and Brockway, U.S. Pat. No. 3,061,472 disclose the use of peroxide and persulfate initiators for the grafting of acrylate esters to gelatinized starch. The starch graft copolymer materials of Brockway, however, are limited in their commercial appeal because of their lack of stability at high solids levels and tendency to separate on storage.
Kightlinger, et al., U.S. Pat. No. 4,301,017 discloses a stable aqueous polymeric dispersion comprised of at least 25% solids by weight of a graft copolymer of at least one vinyl monomer and a derivatized and thinned starch. (Kightlinger, et al., U.S. Pat. No. 4,375,535 is similar and discloses a stable aqueous polymeric dispersion comprised of at least 25% solids by weight of a graft copolymer of at least one vinyl monomer and a derivatized and thinned amylopectin starch.) The Kightlinger patents teach that the stability problems experienced by the Brockway dispersions result, in part, from the nonspecific nature of peroxide and other initiators utilized by Brockway which induce homo- and copolymerization of monomers instead of the desired grafting to starch. The patents disclose that such problems can be minimized or avoided by use of a cerium initiator. After the conclusion of the ceric ion initiated grafting reaction, however, the patent teaches the use of ammonium persulfate and sodium metabisulfite to reduce the level of unreacted monomers by initiating homo- and copolymerization of those monomers.
The Kightlinger patents further disclose the use of a thinned starch derivative disclosed to have a degree of substitution of at least about 0.05 and an intrinsic viscosity of not less than 0.12 deciliters per gram (dl/g). The degree of thinning is taught to affect the tensile properties of the product when used in applications such as paper coatings and the patent states that such properties become much diminished when the intrinsic viscosity of the thinned starch falls below about 0.12 dl/g.
The Kightlinger patents further disclose the use of a dispersion comprising ethyl acrylate/acrylonitrile grafted to starch as a paper coating composition. The acrylate containing starch graft copolymer dispersions disclosed by Kightlinger are limited in their commercial utility, however. While paper coating compositions made up of such starch graft copolymer dispersions are disclosed to provide particularly good qualities (especially gloss) to paper, the acrylate monomers are considered relatively costly.
There exists a desire in the art, therefore, for a stable high solids dispersion having the advantages of the acrylate containing starch graft copolymer dispersions of Kightlinger and for products such as paper coatings and coated papers comprising that product, but which are available at relatively low cost. Attempts have been made, heretofore, to successfully graft monomers which are less costly (and less reactive) than acrylates onto gelatinized starch. Comas, et al., "A Study of Graft Polymers of Wheat Starch and Noncarbohydrate Monomers for Industrial Applications," Stanford Research Institute, Menlo Park, California, Project No. PU-3206 (1963) describes efforts using ceric ion to graft a variety of monomers to gelatinized starch. While attempts to graft monomers such as acrylonitrile, methyl methacrylate and butyl acrylate were generally successful, attempts to treat starch with less reactive monomers such as chloroprene, 1,3-butadiene and styrene resulted in little or no grafting. It is desired that such grafting be relatively efficient because of the negative health and environmental effects of unreacted monomer and the costly equipment and procedures required to recover such unreacted monomer.
Gugliemelli, et al., U.S. Pat. No. 3,984,361 and Gugliemelli, et al., J. Polym. Sci., Polym. Letters Ed., Vol. 14, pp. 215-218 (1976) disclose the successful grafting of chloroprene onto gelatinized starch which they characterize as surprising given the unsuccessful attempts to graft butadiene of Comas, et al. In Gugliemelli, et al., J. App. Polym. Sci., Vol. 23, 635-644 (1979), it was reported that isoprene does not graft onto gelatinized starch by cerium initiation but rather that it requires the presence of an "initiator-monomer" such as acrylonitrile to obtain copolymer side chains. The publication further suggested that other monomers that normally do not graft onto starch by cerium (IV) initiation could be incorporated in starch in graft form by cografting with monomers that normally graft and that have suitable copolymer reactivities. Nevertheless, the art has failed to disclose either grafting of 1,3-butadiene alone or of 1,3-butadiene and other monomers to gelatinized starch at a grafting efficiency sufficiently high as to provide a useful product for applications such as paper coating.